OTTAWA - Controversy over Mike Duffy's Senate housing allowance has mushroomed into a bigger question — whether he's entitled to sit in the Senate at all — that now threatens to ensnare colleague Pamela Wallin.

For the past week, Duffy has been caught up in a controversy that began with questions about potential abuse of the housing allowance and has since grown into a broader constitutional quandary that could soon include his fellow former broadcaster.

The Constitution requires senators to be resident in the provinces they're appointed to represent — but what constitutes residency is not explicitly spelled out.

The Senate's internal economy committee announced Friday that it is seeking legal advice as to whether Duffy, who owns a home in Ottawa, is actually resident in Prince Edward Island, where he owns a cottage.

Committee chairman David Tkachuk said Duffy spends more than 60 days a year in P.E.I., but whether that's sufficient to satisfy the residency requirement is unclear.

"I always thought it was pretty simple, but when you talk to lawyers you find out how complex this really is," Tkachuk said in an interview.

Wallin's name got dragged into the mix Friday by New Democrats in the House of Commons.

Wallin was appointed as a senator for Saskatchewan in December 2008, despite not actually having lived in the province for decades.

Saskatchewan property records suggest she owns a plot of land and co-owns properties with family members in her hometown of Wadena. But one of those records lists her home address in Toronto.

Before becoming enmeshed in the controversy, Tkachuk "always thought (residency means) you live there."

But he admitted his simplistic interpretation of residency was based on his own situation. When he's not in Ottawa for Senate sittings or travelling on Senate business, Tkachuk lives in Saskatchewan, where his wife and grown children reside year-round. He flies back home most weekends.

He never really stopped to consider that not all senators can arrange their residency in the same way, Tkachuk said. For instance, some may move their families to Ottawa so as to avoid long periods of separation from their young children; older senators may not be able to travel to their home provinces as frequently as they'd like; single senators have no spouse to hold down the home fort while they're in Ottawa or travelling.

But should a cottage, where one spends only a few weeks a year, count as residency?

"I think it does and I think it's a question of assessment," said Tkachuk. "In Mike's case, I think last year he spent over 60 days there. He was there a long time, right?"

The ill-defined residency requirement was imposed in the Constitution at a time when air travel was unheard of. Senators would spend months in the capital and return to their home provinces by train only for the summer or lengthy break periods.

Tkachuk said it's time to clarify the residency requirement to take into account modern realities.

"It's a different world, right? I mean, we're not going by a train and spending six months in Ottawa and then coming home ... and people have different careers and people are single. Everybody used to be married," he said.

"We're going to see this as an opportunity to update some of that stuff."

Liberal Sen. Terry Mercer, who sold his home in Ottawa and set up residence in Nova Scotia when he was appointed to represent the province, said he probably spends 60 per cent of his time in Ottawa on Senate business. He believes residency should mean the province in which one pays income taxes, has a driver's licence and is covered by health insurance.

By that standard, Duffy, who applied for a P.E.I. health card earlier this year, would not qualify as resident in the province.

It's debatable whether Wallin would too.

In an email to The Canadian Press, Wallin contended she meets the constitutional requirements to represent Saskatchewan.

"Saskatchewan is my home and I have owned property there for many years. I work hard in Saskatchewan, in Ottawa and across this country to fulfil my duties as a senator," the email said.

Earlier this week, Wallin dodged a CBC question about which province she pays income tax and votes and whether she held a Saskatchewan driver's licence and health card.

Prior to her Senate appointment, Wallin split her time between Toronto and New York working as a corporate director after a long and prominent career as a journalist.

Records kept by Ontario's land registry office show Wallin currently owns a condo unit in Toronto. A spokesman says she also has a parking spot and storage space in the building.

Tkachuk's committee has asked the accounting firm Deloitte to review the residency claims of Duffy, fellow Conservative Senator Patrick Brazeau and Liberal Mac Harb. The accountants will look at their claims about their primary residence and whether they're entitled to a housing allowance to compensate for living expenses while in Ottawa.

Brazeau holds a Quebec seat and Harb, a longtime former Ottawa MP, sits for Ontario.

Brazeau made headlines for very different reasons Friday after he spent the night in jail before appearing in court to face charges of assault and sexual assault. He was released on bail and faces a forced leave of absence from the Senate.

Duffy issued a statement Friday, declaring: "As a Prince Edward Islander, born and bred, I am proud to represent my province and its interests in the Senate of Canada.

"I represent taxpayers with care, and Canadians know I would never do anything to betray the public trust. I have a home in Prince Edward Island as required by law. I will have no further comment until this review is complete."

Property records show Duffy and his wife, Heather, still own a house in Ottawa's west end they bought in July 2003 for $292,783.

Note to readers: This is a corrected story. An earlier version on Feb. 8 erroneously said Wallin owns three Toronto condo units.